Recent years have seen rapid reductions in the size and weight of electronic devices such as mobile telephones, video cameras, and laptop personal computers. This development essentially requires flexible printed circuit boards, which are soft printed circuit boards with flexibility. A flexible printed circuit board is a wiring board that has a thickness of several tens to several hundreds of micrometers and that has high bendability. A flexible printed circuit board thus needs reinforcing for areas in which to mount components such as a connector in order to ensure strength necessary for such mounting. In view of this, a flexible printed circuit board is normally provided with a stiffener fixed with use of a thermosetting adhesive to a surface opposite to the component mounting areas. This can locally improve the strength of the flexible printed circuit board (see, for example, Patent Literatures 1 to 3).
A flexible printed circuit board is also provided with an insulator film serving as a surface protecting material. Regarding such an insulator film, there has been a technique of giving the black color to an insulator film to impart concealment property to the insulator film, thereby concealing the circuit pattern to protect confidential information included in the circuit pattern (see, for example, Patent Literatures 4 and 5).